Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table. Helium is used in cryogenics (its largest single use, accounting for about a quarter of production), and the cooling of superconducting magnets, particularly the main commercial application in MRI scanners. Helium's other industrial uses as a pressurizing and purge gas, and a protective atmosphere for arc welding and processes (such as growing crystals to make silicon wafers), account for half of its use. Economically minor uses, such as lifting gas in balloons and airships are popularly known. As with any gas with differing density from air, inhaling a small volume of helium temporarily changes the timbre and quality of the human voice.